(i) Field of the Invention
This invention relates to systems for minimizing heat loss from buildings. More particularly, it is directed to systems for selectively insulating large transparent exterior walls, e.g., the roofs and walls of greenhouses and other structures., e.g., swimming pool covers.
(ii) Description of General Problem
There are in the United States and Canada commercial greenhouses totalling over 250 million square feet. Over two-thirds of this total is in the northern United States and Canada, where greenhouses must be heated during at least some of the year. The greenhouse heating cost exceeds $120 million annually.
Most of the fuel used to heat greenhouses is expended at night. It is thought that night-time heating accounts for as much as 80% of the total heating bill. Recent increases in fuel cost, coupled with severe shortages of fuels, have faced greenhouse owners with a major problem. They must cut their total fuel consumption drastically if they wish to remain competitively viable.
The energy requirements of commercial greenhouses for heating are so high at present as to exclude economic operation of many greenhouses. One way to cut fuel consumption is to reduce the heat loss through the greenhouse walls and roof, especially at night. Insulating the walls or roof can significantly reduce heating requirements, but itself presents several potentially serious problems. An insulating system should not significantly reduce the amount of sunlight that can enter the house during the day. The insulating system may take up valuable growing space within the greenhouse. North walls and part of roofs have been insulated, but these reduce effective growing area and the production per plant because of poor light, to the point where the structure is not economic. The radiant heat loss at night can be minimized by using cloth or polyethylene screens inside the structure and these have been attempted. They fail in cold climates (i.e., below -18.degree. C.) because the moisture-laden air creeps around the screen and forms frost on the inside cover of the greenhouse. Often this falls off when the sun warms the roof and walls, thereby damaging the plants as well as creating a hazard for workers. If it does not, the resulting chill from the cold air above the screen reduces the crop yield significantly (e.g., 10 to 15%). Outside shutters have also been tried but wind problems and economics exclude this type of arrangement in commercial greenhouses although feasible in back yard greenhouses. Accordingly, an insulating system should be readily adaptable for use with greenhouses of widely varying size and construction, and should cover a large area rather than requiring many small systems installed between each pair of obstructing greenhouse supports.
(iii) Description of the Prior Art
Many systems have been proposed to control the temperature within greenhouses. Thus, French Patent No. 371,926 dated Mar. 19, 1907 provides a system in which shades are provided which can be rolled up or down the exterior arched transparent walls of greenhouse structures.
Italian Patent No. 695,829 dated Sept. 27, 1965 provides a system in which shades are drawn within the inside of the greenhouse structure, the shades being in the form of a movable horizontal ceiling.
Italian Patent No. 717,643 dated Oct. 15, 1966 provides a system in which shades are drawn across the transparent portion of the greenhouse parallel to the sloping roof thereof to provide an internal, dropped shielding ceiling.
Canadian Patent No. 982,426 issued Jan. 27, 1976 to R. Delano et al provides a method of protecting greenhouses involving coating the inside transparent surfaces of the greenhouses with a coating which is translucent, so that the amount of light permitted therethrough is controlled by the thickness and density of the coating and which, when wet, becomes almost transparent, permitting the passage of considerably more light therethrough than when dry. The humidity of the interior of the greenhouse, which was alleged to vary from high on cloudy days to low on sunny days, was said automatically to control the amount of light entering the greenhouse.
Canadian Patent No. 1,003,641 issued Jan. 18, 1977 to H. Grossman et al provided a shade-providing system including at least one powered track on which a drape or shade cloth was supported for covering an area. The powered track included a self-contained motor means having a track guiding means for drawing the shade cloth between a gathered stored position and to an extended position for shading a selective area. The shade system could also include suspension tracks spaced from the powered track means for maintaining the shade cloth or panel elevated above the benches. The suspension tracks were said to be devoid of any actuating means and served merely as a support and guide for the shade cloth.
U.S. Pat. No. 4,064,648 issued Dec. 27, 1977 to C. L. Cary provided an insulating system for reducing heat loss from a structure during one part of the day and for permitting light to enter the structure during another part of the day. This system included a roll mounted within the structure, a flexible sheet of material wound around the roll, a structure for supporting the roll immediate its length and engaging portions of the wound sheet, and means both for unwinding the sheet from the roll and deploying it in a plane and for rotating the roll to rewind the sheet therearound.
Canadian Patent No. 1,043,070 issued Nov. 28, 1978 to M. Dube provided a system of filling double-glazed building panels with insulating light-weight granular material for the purpose of providing insulating shading or privacy and for evacuating such material therefrom when light transmission was to be restored. The system included a container for the material, a header for through-flow of gas-conveyed granular material into and from the space between the double-glazed unit, and blower means and controls therefor for conveying the granular material into the space and for retaining it therein by gaseous pressure.
U.S. Pat. No. 4,067,347 issued Jan. 10, 1978 to Lipinski provided a portable solar-heated shelter comprising at least one fixed roof layer and a second mechanically supported roof layer which can be selectively employed to vary the thermal characteristics of the shelter. The second roof layer was adapted to be unwound from a storage spool and drawn into a take-up spool, passing over the first roof layer, by a cranking action. The second roof layer included a first sunshade portion and a series connected heat insulative portion of opaque material which may be selectively deployed to control the thermal characteristics of the enclosure.
Canadian Patent No. 1,054,081 issued May 8, 1979 to D. M. Fraioll provided a double wall fabric panel unit supported by pressurized air pumped into the interior thereof, with insulation provided in the double walled panels by including a plastic coated fabric panel and a thermal liner panel, with said edge strips being discontinuous to provide spaced air passageways to vent air from between the panels as the unit is rolled.
While the teachings of U.S. Pat. Nos. 4,067,347 (Lipinski) and 4,064,648 (Cary), described above, provided a movable insulation to allow entrance of light in day and provide insulation at night, the Lipinski and Cary approaches are uneconomical because of their complexity and have one basic failing, namely, that they do not protect against formation of frost or ice within the structure but outside of the insulating layer.
Moreover, Cary does not provide a movable insulation outside the usable space. The system taught by Cary is very difficult and expensive to install. Installed within a greenhouse, it will be extremely difficult to overcome the infiltration of warm, moist air above the flexible material at night which will freeze on the mechanism or the roof. In cold climates (e.g., as in Canada and the northern United States), this system will not prevent the freezing problem, and in addition, it will create a cold mass of air above the blanket that will fall once the blanket is withdrawn, and result in chilling that will impede the growth of greenhouse crops. Cary attempts to overcome the icing and snow accumulation on the outside of the roof by automatically retracting the blanket.
Lipinski provides for the placement of the insulating barrier between unpressurized flexible roof layers. The system, however, will fail in cold freezing climates since any small hole (either accidental or otherwise) will allow the warm moist air to penetrate into the space between his roofs, freeze onto the mechanism, tear the walls and immobilize the blanket. Its very nature only allows it to be used on structures of short length, i.e., movable shelters.